1. Field of the Invention
The present invention relates to a method of controlling a fuel cell vehicle and a fuel cell system, each having a load energizable by electric power that is generated by a fuel cell from hydrogen supplied from a hydrogen storage device and an oxygen-containing gas supplied from an air pump, and more particularly to a method of controlling a fuel cell vehicle and a fuel cell system of the type described above to prevent the fuel cell from being deteriorated and also to prevent the user from feeling uncomfortable about the output sound generated by the air pump when electric power demanded by the load drops in level.
2. Description of the Related Art
In recent years, intensive efforts have been made to carry out research and development on fuel cells. A fuel cell generates electric power when anodes thereof are supplied with hydrogen and cathodes thereof are supplied with an oxygen-containing gas. It is known in the art that the hydrogen is supplied to the anodes from a hydrogen storage device which stores hydrogen under high pressure and the oxygen-containing gas is supplied to the cathodes from an air pump or an air compressor (see, for example, Japanese Laid-Open Patent Publication No. 2007-149496 and Japanese Laid-Open Patent Publication No. 2002-352826).
According to Japanese Laid-Open Patent Publication No. 2007-149496, the flow rate of the oxygen-containing gas is regulated by an air compressor (36), and the flow rate of the fuel gas is regulated depending on the pressure of the oxygen-containing gas (see, for example, paragraphs [0014], [0016] of the publication). The flow rates of the oxygen-containing gas and the fuel gas are similarly regulated according to Japanese Laid-Open Patent Publication No. 2002-352826 (see, for example, paragraphs [0013], [0017] of the publication). According to Japanese Laid-Open Patent Publication No. 2002-352826, furthermore, a target generated current (IFC) for a fuel cell (11) is calculated depending on an accelerator opening (AC), and a target flow rate (QM) and a target pressure (PM) for a reaction gas are calculated depending on the target generated current. In addition, a rotational speed (N) of an air compressor (21) is calculated based on the target generated current for controlling the air compressor (see, for example, FIG. 2, and paragraphs [0031], [0033] of the publication).
It is also known in the art that fuel cells are used as vehicle power sources (see, for example, U.S. Patent Application Publication No. 2006/0021812 and Japanese Laid-Open Patent Publication No. 2009-032418). According to U.S. Patent Application Publication No. 2006/0021812, electric power is supplied from one or both of a fuel cell and a battery to a traction motor to energize the traction motor (see, for example, paragraph [0040] of the publication). According to Japanese Laid-Open Patent Publication No. 2009-032418, for the purpose of preventing a fuel cell from being deteriorated, the rate of change of a cell voltage is controlled so as to be equal to or lower than a predetermined value when the load on the fuel cell changes from a high level to a low level (see, for example, the abstract of the publication).
If the control process disclosed in Japanese Laid-Open Patent Publication No. 2009-032418 is applied to the fuel cell control apparatus disclosed in Japanese Laid-Open Patent Publication No. 2002-352826 which determines the rotational speed of the air pump depending on the accelerator opening, i.e., the magnitude of the load, then the user may possibly find the output sound generated by the air pump uncomfortable though the fuel cell itself is prevented from being deteriorated.
More specifically, according to the control process disclosed in Japanese Laid-Open Patent Publication No. 2009-032418, since the rate of change of the cell voltage is reduced to a predetermined value or lower, the rate of change of a cell current is also reduced due to the current-voltage characteristics of the fuel cell. As a result, the electric power generated by the fuel cell drops at a rate which is lower than the rate at which the electric power demanded by the load in response to the accelerator opening drops. In the control process disclosed in Japanese Laid-Open Patent Publication No. 2009-032418, if the rotational speed of the air pump is related to the electric power generated by the fuel cell, then even when the electric power demanded by the load is lowered, the rotational speed of the air pump is not reduced correspondingly, and the output sound generated by the air pump may possibly make the user feel uncomfortable.